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1.2.2.7.2 Using Shiʼs Ketone

DOI: 10.1055/sos-SD-201-00018

Matsumoto, K.; Katsuki, T.; Arends, I. W. C. E.Science of Synthesis: Stereoselective Synthesis, (20111105.

The most reliable ketone catalyst is Shiʼs ketone (74; Scheme 51),[‌123‌,‌124‌] which can be prepared in two steps from d-fructose. Epoxidation with this catalyst is quite effective for E-disubstituted and trisubstituted alkenes, and high enantioselectivity (generally >90% ee) is obtained in the presence of typically 1030mol% catalyst (Scheme 51). Various functional groups, such as ethers, esters, and acetals, remain unaffected under the reaction conditions. Allylic and homoallylic alcohols are also good substrates for the epoxidation, as the hydroxy group is also compatible with the reaction conditions.[‌125‌] High enantioselectivities can be achieved with dienes, enynes, alkenylsilanes, or enol esters.[‌126‌‌129‌] The major problem in ketone-catalyzed epoxidation is decomposition of the catalyst by BaeyerVilliger oxidation, which diminishes the conversion of the substrate. The decomposition process is markedly affected by the pH of the reaction medium, but the enantioselectivity is unaffected.[‌130‌] Whereas rapid decomposition occurs when the reaction is carried out at pH78, higher pH values suppress the decomposition and improve the substrate conversion. Thus, the epoxidation is typically performed at around pH10.5, and the pH is controlled by the addition of a base such as potassium carbonate or potassium hydroxide. To obtain the desired epoxides in high yield, it is important to maintain a constant pH throughout the reaction, so the epoxidation is carried out under buffered conditions.

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